1. Field of the Invention
The present invention relates to a band allocation method in a system for dividing a variable-length packet into cells, superimposing the cells on a time slot and transmitting the time slot and a transmission system realizing the method.
2. Description of Related Art
A transmission system is widely known in which a plurality of terminal units are connected to a network unit using branched optical transmission lines, etc., polling information is transmitted according to polling request information sent from a terminal unit and information divided into cells is transmitted from a terminal unit designated by the polling information. In this system, a band can be allocated to a terminal unit using polling information from a network unit. Such a band allocation is strongly desired.
FIG. 1 shows the rough configuration of a conventional transmission system, and in FIG. 1, a plurality of terminal units 52-1 through 52-n are connected to a network unit 51 via an optical transmission line 53. Specifically, this system comprises a multiplexer/demultiplexer 54 for connecting an optical transmission line 53 from the network unit 51 and optical transmission lines from the plurality of terminal units 52-1 through 52-n, and has a so-called PON (Passive Optical Network) or PDS (Passive Double Star) configuration. The terminal units 52-1 through 52-n correspond to subscriber line terminating units on the subscriber side, and a telephone set, facsimile transmitter/receiver, personal computer, etc., are connected to the terminal units 52-1 through 52-n.
The network unit 51 comprises an identification unit 55 for identifying polling request information, a control unit 56 and a creation unit 57 for creating polling information. The terminal units 52-1 through 52-n comprise a buffer 58 for temporarily storing transmission information and a request creation unit 59 for creating polling request information. A transmitter/receiver unit, optical/electrical (O/E) converter unit, etc., possessed by both the network unit 51 and terminal units 52-1 through 52-n, are not shown in FIG. 1 for simplicity.
When transmission information is stored in the buffer 58, the terminal units 52-1 through 52-n create polling request information using the request creation unit 59, and the polling request information is transmitted to the network unit 51. The network unit 51 receives the polling request information from a terminal unit, identifies the terminal unit which has transmitted the polling information using the identification unit 55, creates polling information using the creation unit 57 under the control of the control unit 56 and transmits the polling information to the terminal unit which has transmitted the polling request information.
The terminal units 52-1 through 52-n superimposes an information cell on the prescribed time slot of the polling information from the network unit 51 and transmits the information cell to the network unit 51. In this case, the network unit 51 can allocate a wide band to a prescribed terminal unit, for example, by repeatedly transmitting polling information to the terminal unit in response to the polling request information from a plurality of terminal units.
A method in which the network unit 51 sets an individual management time slot, makes the terminal units 52-1 through 52-n transmit information about the amount of transmission information stored in the buffer 58, executes the allocation process of a time slot in the next frame based on information about the amount of storage of a buffer and transmits polling information to the terminal units 52-1 through 52-n corresponding to the time slot allocation, is widely known (for example, see Japanese Patent Laid-open No. 10-107755).
A method for executing the time slot allocation process according to the amount of storage (amount of information queued) of a buffer, by comparing the amount of storage with a threshold value or according to the priority among the terminal units 52-1 through 52-n, and transmitting polling information to the terminal units 52-1 through 52-n accordingly, is also known (for example, Japanese Patent Laid-open No. 10-242981).
As described earlier, according to the conventional method, the network unit 51 transmits polling information based on band allocation carried out based on the amount of storage of a buffer of the terminal units 52-1 through 52-n, and each of the terminal units 52-1 through 52-n divides a frame or packet into cells and transmits the cells according to polling information from the network unit 51. Therefore, the network unit 51 can receive and process cells from the plurality of terminal units 52-1 through 52-n without collision.
According to a system for dividing a frame or packet into a plurality of cells and transmitting the cells, whether all the cells can be received is judged by assembling received cells into the frame or packet. During this time, cells composing the next frame or packet arrive. Therefore, if even one of the cells composing a frame or packet is lost, the frame or packet must be transmitted again. However, since cells composing the next frame or packet have already arrived, the re-transmission process becomes complicated, which was one problem. If a band is allocated based on only the amount of storage of a buffer, as in the conventional method and, for example, if the amount of storage of a buffer of a packet with a short packet length and the amount of storage of a buffer of a packet with a long packet length are the same, the same band is allocated to both the long and short packets, and cells are allocated to time slots in the same way in both cases. Therefore, in a system for transmitting data in units of packets, a packet cannot be efficiently divided into cells, multiplexed and transmitted, which was another problem.
The objective of the present invention is to carry out an efficient band allocation and to improve transmission efficiency when a variable-length packet is transmitted.
A method for allocating a band to transmit a variable-length packet, which is one of the aspects of the present invention, assumes a method for allocating a band to transmit a variable-length packet in a system where a plurality of terminal units connected to a network unit divide a variable-length packet into cells and transmit the cells according to polling information from the network unit.
According to this aspect of the present invention, the terminal units transmit polling request information including information about the number of cells composing a variable-length packet stored in a buffer, to a network unit, whereas the network unit allocates in order a time slot to the plurality of terminal units in units of variable-length packets according to the polling information and transmits polling information based on the allocated time slot.
The terminal units can also be designed so as to transmit polling request information including information about both the number of cells remaining in a buffer, composing a variable-length packet being read from the buffers by the terminal units and the number of cells composing the next variable-length packet to be transmitted.
The network unit can also be designed so as to allocate in order a time slot to a terminal unit in which a cell remains, so as to allocate in order a time slot to the terminal unit in units of cells of the next variable-length packet to be transmitted when the number of remaining cells becomes zero.
The system can also be configured in such a way that the polling request information further includes information indicating the service class of a variable-length packet stored in a buffer and that the network unit carries out with priority the time slot allocation based on the number of cells corresponding to the service class of a variable-length packet.
Alternatively, the network unit can be designed so as to allocate with priority a time slot based on a weight pre-set in each terminal unit.
A transmission system, which is another aspect of the present invention, assumes a transmission system in which a plurality of terminal units connected to a network unit divide a variable-length packet into cells and transmit the cells according to polling information from the network unit.
This aspect of the present invention is configured in such a way that each terminal unit comprises a buffer for temporarily storing variable-length packets, a polling request information creation unit for creating polling information including information about the number of cells composing each variable-length packet in the buffer and the number of cells remaining in the buffer, out of the cells composing a variable-length packet being read from the buffer in units of cells, and a polling information extraction unit for extracting polling information addressed to its own terminal unit from signals received from a network unit and making the terminal unit read the variable-length packet from the buffer in units of cells according the polling information. This aspect of the present invention is also configured in such a way that the network unit comprises a shared band control unit for allocating in order a time slot to each terminal unit according to information about the number of cells composing a variable-length packet, included in polling request information received from a plurality of terminal units.
The system can also be configured in such a way that the polling request information creation unit creates polling request information further including information about the service class of a variable-length packet in the buffer and in such a way that the shared band control unit allocates a time slot with priority according to the service class of a variable-length packet included in the polling request information.
The shared band control unit can also be designed so as to allocate a time slot to the terminal units based on a weight pre-set in each terminal unit.
The system can also be configured in such a way that the network unit further comprises a pre-assignment polling control unit for periodically transmitting polling information to a plurality of terminal units and in such a way that the shared band control unit allocates a time slot in a non-polling time area, which is a time period when polling information is not transmitted from the pre-assignment polling control unit.
The system can also be configured in such a way that the buffer stores a variable-length packet for each service class set in the variable-length packet, and a variable-length packet stored in the buffer is read in units of cells in the order of the service class according to the polling information extracted from the polling extraction unit and the variable-length packet is transmitted to the network unit.
Alternatively, the system can be configured in such a way that the buffer stores a variable-length packet for each service class set in the variable-length packet, the variable-length packet stored in the buffer is read in units of cells, the variable-length packet is transmitted to the network unit according to the polling information extracted by the polling information extraction unit, and another variable-length packet stored in the buffer is read in units of cells and transmitted when the number of remaining cells of one variable-length packet being read from the buffer becomes zero.
In any of the aspects of the present invention described above, a variable-length packet can be divided into cells in units of variable-length packets and transmitted. For example, if the packet length of a variable-length packet is long, that is, if the number of cells produced by dividing a variable-length packet is large, many time slots can be allocated to the packet. Therefore, even if a re-transmission process is required, data can be efficiently transmitted in units of variable-length packets.